The present invention relates to the production of a superconductive composite metal oxide film for application in a superconductive microwave device, a fault current limiter, a tape, a wire and the like.
Various methods for forming an oxide superconductor film have been developed.
Among these methods, there is a metal organic deposition, which uses, as the raw material, a solution containing an organic compound including an atomic species which forms a superconductive film on various types of substrate materials, coats the foregoing solution on a substrate, performs heat treatment so as to thermally decompose the coated film, and thereby forms a superconductive film. With this method, the following are demanded; namely, to dissolve metal organic compounds containing atomic species which comprise superconductive composite metal oxide in a solvent as homogeneously as possible to prepare a homogeneous mixed solution, to coat this solution evenly on a substrate material, to perform heat treatment and subject the metal organic substance to pyrolysis treatment so as to eliminate only the organic components, and to perform high temperature heat treatment so as to uniformly form a superconductive film via solid-phase reaction or liquid-phase reaction. The present inventors have been actively involved in the development of the foregoing method, and devised inventions pertaining to a method of producing a superconductive film, and a coating solution (Patent Documents 1 and 2).
Known is an invention by Kumagai et al. in relation to the method of using low oxygen partial pressure or decompression upon performing high temperature heat treatment (Patent Documents 3 and 4). This method is characterized in an inexpensive production method compared to a vacuum evaporation method or the like, since a vacuum device is not required, and is also characterized in that deposition onto a long or large-area substrate is easy. This production method was highly appreciated from the perspective that the characteristics of the superconductive film produced with said method are also favorable compared to other production methods.
Encouraged by the success of forming superconductive films based on the coating-pyrolysis method, research and development concerning the preparation of superconductive films using similar methods were actively conducted in organizations throughout the world, and the following methods have been publicly announced.
U.S. IBM Thomas J. Watson Research Center, and subsequently Massachusetts Institute of Technology announced that a superconductor can be formed by coating a trifluoroacetate solution on a substrate material, and performing heat treatment thereto in a water-vapor atmosphere (Non-Patent Documents 1 and 2). Subsequently, Superconductivity Research Laboratory announced that it improved and optimized the foregoing process and succeeded in preparing a superconductive film having high critical current characteristics (Non-Patent Document 3). Upon using the foregoing organic compound containing fluorine as the starting raw material, high critical current characteristics can be obtained, but there is a problem in that hydrogen fluoride, which is toxic and environmentally burdensome, is generated during the heat treatment.
Meanwhile, when metal organic acid salt or acetylacetonate which does not contain fluorine is used as the raw material, hydrogen fluoride is not generated during the heat treatment. Nevertheless, when applying this method to usage which requires a thick film of an oxide superconductor such as with a superconductive microwave device, a tape or a wire, upon coating a metal organic compound solution on a substrate and performing a heat treatment process to eliminate the organic components, cracks will occur in the heat treatment process of eliminating the organic components if the thickness of a single-coat film which is expressed as the thickness of the produced oxide superconductor film (“single-coat equivalent film thickness” hereinafter) becomes 500 nm or more. Since these cracks cannot be mended even with the subsequent high temperature heat treatment, the characteristics of the ultimately formed superconductive film will deteriorate considerably. Thus, in order to avoid the generation of cracks in the heat treatment process of eliminating the organic components, the coating and pyrolysis process need to be repeated so that a single-coat equivalent film thickness is reduced to roughly 100 to 300 nm, and a problem of increase in time, work and required energy will arise.
Thus, in order to increase the film thickness of a single coating, proposed is increasing the viscosity by adding a macromolecular additive agent such as polyvinyl butyral, polyethylene glycol, or polyvinyl pyrrolidone to the solution. Nevertheless, under the current circumstances, a thickness of up to only 400 nm can be achieved (Non-Patent Document 4).